A Presentation on DC Generator

DC Machine Construction General arrangement of a dc machine

DC Machines The stator of the dc motor has poles, which are excited by dc current to produce magnetic fields. In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current. Compensating windings are mounted on the main poles. These short-circuited windings damp rotor oscillations. .

DC Machines The poles are mounted on an iron core that provides a closed magnetic circuit. The motor housing supports the iron core, the brushes and the bearings. The rotor has a ring-shaped laminated iron core with slots. Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees.

DC Machines The coils are connected in series through the commutator segments. The ends of each coil are connected to a commutator segment. The commutator consists of insulated copper segments mounted on an insulated tube. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

DC Machines The rotor has a ring-shaped laminated iron core with slots. The commutator consists of insulated copper segments mounted on an insulated tube. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

DC Machines The commutator switches the current from one rotor coil to the adjacent coil, The switching requires the interruption of the coil current. The sudden interruption of an inductive current generates high voltages . The high voltage produces flashover and arcing between the commutator segment and the brush.

DC Machine Construction Commutator with the rotor coils connections.

DC Generator Operation

DC Generator Operation The N-S poles produce a dc magnetic field and the rotor coil turns in this field. A turbine or other machine drives the rotor. The conductors in the slots cut the magnetic flux lines, which induce voltage in the rotor coils. The coil has two sides: one is placed in slot a, the other in slot b. (a) Rotor current flow from segment 1 to 2 (slot a to b) (b) Rotor current flow from segment 2 to 1 (slot b to a)

DC Generator Operation In Figure, the conductors in slot a are cutting the field lines entering into the rotor from the north pole, The conductors in slot b are cutting the field lines exiting from the rotor to the south pole. The cutting of the field lines generates voltage in the conductors. The voltages generated in the two sides of the coil are added. (a) Rotor current flow from segment 1 to 2 (slot a to b) (b) Rotor current flow from segment 2 to 1 (slot b to a)

DC Generator Operation The induced voltage is connected to the generator terminals through the commutator and brushes. In Figure, the induced voltage in b is positive, and in a is negative. The positive terminal is connected to commutator segment 2 and to the conductors in slot b. The negative terminal is connected to segment 1 and to the conductors in slot a. (a) Rotor current flow from segment 1 to 2 (slot a to b) (b) Rotor current flow from segment 2 to 1 (slot b to a)

DC Generator Operation When the coil passes the neutral zone: Conductors in slot a are then moving toward the south pole and cut flux lines exiting from the rotor Conductors in slot b cut the flux lines entering the in slot b. This changes the polarity of the induced voltage in the coil. The voltage induced in a is now positive, and in b is negative. (a) Rotor current flow from segment 1 to 2 (slot a to b) (b) Rotor current flow from segment 2 to 1 (slot b to a)

DC Generator Operation The simultaneously the commutator reverses its terminals, which assures that the output voltage (Vdc) polarity is unchanged. In Figure the positive terminal is connected to commutator segment 1 and to the conductors in slot a. The negative terminal is connected to segment 2 and to the conductors in slot b. (a) Rotor current flow from segment 1 to 2 (slot a to b) (b) Rotor current flow from segment 2 to 1 (slot b to a)